Convert an RGB

[Ofnuts]

• Start the Levels tool.
  
• Set the Channel selector to "Green"
  
• In the Input levels (just below the histogram) drag the rightmost handle to value 85. This will spread the input range 0-85 (85≈256/3) over the 0-255 output range.
  

However this may not be completely valid. If the colors of the darker greens seems to be a bit off:

• click the "Edit these settings as Curves" button,
  
• Set the Channel selector to "Green"
  
• Click the middle of the green slope
  
• Drag it up or down